When a data pulse is applied to a laser diode such as in an optical communication system, the laser diode switches between an OFF condition, where very little current flows through the diode and there is no substantial lightwave or optical output signal, and an ON condition, where a large amount of current flows through the laser diode to cause it to lase and produce an optical output signal. As the laser changes between the OFF condition and the ON condition, a shift occurs in wavelength. This shift is commonly called "chirp." This wavelength shift or chirp causes the optical pulse to disperse or smear in time as the pulse travels through the optical fiber. This phenomenon makes it difficult to distinguish between a logic 0 level and a logic 1 level in the signal at the optical receiver. This difficulty may be termed "bit-error rate degradation."
In high data rate optical communication systems, laser chirp characteristics impose a significant limitation on the maximum obtainable transmission distance. In order to minimize laser chirp, the amplitude of the non-return-to-zero (NRZ) data modulation applied to the laser must be controlled to avoid driving the NRZ data logic zero level into the laser threshold region. This restriction prevents the use of known laser threshold sensing modulation control techniques that are used at lower data rates.
Existing approaches for preventing bit-error rate degradation due to laser chirp in high-speed optical communication systems include (1) techniques that use laser threshold sensing; (2) techniques that add a low-frequency control tone to the NRZ data logic 1; and (3) approaches that monitor and measure the laser gain at the laser diode back facet. These solutions have significant limitations as the following discussions explain.
Threshold sensing techniques are unsatisfactory because they require the laser diode to operate in the threshold region. Laser operation in the threshold region, however, inherently produces excessive chirp. This approach, therefore, is an unsatisfactory way to prevent bit-error rate degradation.
Adding a low-frequency tone to the logic 1 of the NRZ data stream in high speed systems does not work well, because it is difficult to add a low-frequency tone to only one side of the NRZ data stream. This technique is also expensive, at least in part because it requires the use of radiofrequency devices for logic 1 signal level manipulations. Reliability is also less than desirable in techniques that add a low-frequency tone to the logic 1, because the radiofrequency devices that can accomplish this detection are fragile. These techniques also generally require a good phase match between the primary high speed data path and the secondary logic/manipulation path over the communication system temperature and voltage operating ranges. This phase match requirement even further adds to the difficulty of accomplishing this technique.
Existing techniques that measure the laser gain by monitoring the high speed data with a back facet monitor do not work well for numerous reasons. One reason is that back facet detectors generally have limited bandwidth. Also, the amount of high speed signal data at the laser back facet that is available for detection is small. Back facet control systems that monitor these high speed signals, therefore, must recover a small signal and attempt to monitor signals that may have frequencies of up to 1.2 GHz. These techniques require a significant amount of broadband gain to recover the NRZ data from the laser diode back facet signal. In addition, these techniques require coupling high speed electronic circuits with the laser diode back facet. High speed electronic circuits that can couple with the laser back facet, furthermore, are generally expensive and unreliable. Due to the broadband nature of this technique, inherent detector limitations, and the type of signal that this approach detects, existing back facet control systems do not produce a sufficiently high signal-to-noise ratio to ensure stable modulation level control.
Gain stability over normal operating temperature and voltage ranges is yet another difficulty that existing back facet control systems employing these techniques experience. That is, as temperature or voltage changes, drift in the NRZ signal level occurs due to gain changes in the associated broadband amplifiers. This gain instability adversely affects detector accuracies. As the drift increases, so do detector inaccuracies. These inaccuracies also make monitoring high speed data to measure laser gain an unacceptable technique.
Consequently, there is a need for method and circuit that control the modulation of a laser that provides communication signals while the laser operates in its linear region.
There is a need for a laser modulation controller that does not require a significant bandwidth signal from the laser back facet.
There is a further need for a laser modulation controller that does not require expensive radiofrequency or other high speed electronic detection circuits or circuits that are highly sensitive to optical communication system temperature or voltage changes.